1. Field of the Invention
The present invention relates to a method for producing a magnetic recording medium comprising a two-layered magnetic layer and having an excellent high density recording property.
2. Description of Related Art
As a recording density of a magnetic recording/reproducing equipment has been increased year by year, it is highly desired to provide a magnetic recording medium which is excellent in recording and reproducing characteristics in a short wavelength range. Today, a coating type magnetic recording medium in which magnetic powder is coated on a substrate is mainly used, and its properties are being improved to satisfy the above desire. However, the improvement of the properties is nearing its limit.
One of magnetic recording media which can exceed this limit is a thin film magnetic recording medium. The thin film magnetic recording medium is produced by a vacuum deposition method, a sputtering method or a plating method and has excellent recording and reproducing characteristics in a short wavelength range. Examples of magnetic materials used in the thin film magnetic recording medium are Co, Co-Ni, Co-Ni-P, CO-O, CO-Ni-O, Co-Cr, Co-Ni-Cr, Co-Cr-Ta, Co-Cr-Pt and the like.
It is supposed that Co-O and Co-Ni-O are most suitable for the practical use as the magnetic recording tape, and a deposition cape comprising a Co-Ni-O magnetic layer is already practically used as a Hi-8 type video tape. A magnetic property of an oxygen-containing magnetic layer widely varies depending on an amount of supplied oxygen at the deposition. One embodiment of the production method of a deposition Hi-8 type video tape will be explained by making reference to FIG. 1.
FIG. 1 schematically illustrates an inner structure of a vacuum deposition apparatus conventionally used for producing the deposition Hi-8 type video tape. A substrate 1 made of a polymer film travels around a peripheral surface of a cylindrical can 4 in a direction of an arrow A. A vapor 3 of a raw magnetic material 6 is evaporated from an evaporation source 5 receiving the raw material 6, and is deposited on the substrate 1 to form a magnetic layer on the substrate 1. As the evaporation source 5, an electron beam evaporation source is preferred, and the raw material 6 such as a metal, for example, CO and an alloy, for example, Co-Ni is filled in the evaporation source S. The reason why the electron beam evaporation source is used is that it can evaporate a high melting point metal such as Co at a high evaporation rate.
Below a part of the circumference of the cylindrical can 4, there is provided a shielding plate 8 which prevents excessive deposition of the evaporated atoms on the substrate.
An oxygen gas is supplied in a vacuum chamber from nozzle 9 positioned near the end of shielding plate 8 during the vapor deposition o-f the evaporated atoms on the substrate. The optimization of the position of the nozzle 9 and the amount of the supplied oxygen gives a deposition tape having an excellent recording/reproducing property and excellent practicability. The substrate 1 is unwound from the supply roll 2 and, after the formation of the magnetic layer, is wound on the wind-up roll 3.
In future, the magnetic tape will be required to have a higher output and a lower noise in a short wave length range. As one of methods to satisfy this requirement, it is thought to use a magnetic layer having a two-layered structure.
However, when the vacuum deposition apparatus shown in FIG. 1 is used to form a magnetic layer having a two-layered structure, a step comprising depositing the magnetic layer on the substrate during the travel of the substrate 1 must be conducted twice. When the conventional method is used to form the magnetic layer having the two-layered structure by only one travel of the substrate, it is necessary to use a vacuum deposition apparatus equipped with two cylindrical cans. When these methods are used to form the magnetic layer, the recording/reproducing property is improved but any of these methods requires a high cost.